Raphide

Raphides are needle-shaped crystals of calcium oxalate monohydrate or calcium carbonate as aragonite, found in more than 200 families of plants.
Both ends appear needle-like, but raphides tend to be blunt at one end and sharp at the other.

Calcium oxalate in plants

Many plants accumulate calcium oxalate crystals in response to surplus calcium, which is found throughout the natural environment. The crystals are produced in a variety of shapes. The crystal morphology depends on the taxonomic group of the plant. In one study of over 100 species, it was found that calcium oxalate accounted for 6.3% of plant dry weight. Crystal morphology and the distribution of raphides is similar in some taxa but different in others leaving possible opportunities for plant key characteristics and systematic identification; mucilage in raphide containing cells makes light microscopy difficult, though. Little is known about the mechanisms of sequestration or indeed the reason for the accumulation of raphides but it is most likely as a defense mechanism against herbivory. It has also been suggested that in some cases raphides may help form plant skeletal structures. Raphides typically occur in parenchyma cells in aerial organs, especially the leaves, and are generally confined to the mesophyll. As the leaf area increases, so does the number of raphides, the process starting in even young leaves. The first indication that the cell will contain crystals is shown when the cells enlarge with a larger nucleus.
Raphides are found in specialized plant cells or crystal chambers called idioblasts. Electron micrographs have shown that raphide needle crystals are normally four sided or H-shaped or with a hexagonal cross section and some are barbed. Wattendorf suggested that all circular sectioned raphides, as visible in a light microscope, are probably hexagonal in cross section The hexagonal crystals reported by Wattendorf in Agave americana were apparently calcium oxalate.
Microscopy using polarized light shows bright opalescence with raphides.
Plants like Tradescantia pallida also accumulate calcium oxalate crystals in response to heavy metals stress.

Harmful effects

Raphides can produce severe toxic reactions by facilitating the passage of toxin through the herbivore's skin when the tissue containing the raphides also contains toxins. The lethal dose to mice is around 15 mg/kg. Raphides seem to be a defense mechanism against plant consuming animals, as they are likely to tear and harm the soft tissues of the throat or esophagus of an animal chewing on the plant's leaves. The venomous process is in two stages: mechanical pricking and injection of harmful protease. Typically ingestion of plants containing raphides, like those common in certain houseplants, can cause immediate numbing followed shortly by painful edema, vesicle formation and dysphagia accompanied by painful stinging and burning to the mouth and throat with symptoms occurring for up to two weeks. Airway assessment and management are of the highest priority, as are extensive irrigation and analgesics in eye exposure.
Raphides cannot normally be destroyed by boiling; that requires an acidic environment or chemical solvents like ether. Plants containing large amounts of raphides are generally acrid and unpalatable. However, it is not always possible to detect the presence of raphides through taste alone. In some tubers such as the Indian turnip which contain large quantities of raphides, the roots are not unpalatable when cooked because the raphides are bound within a matrix of starch which prevents the tongue from sensing their presence. Some other plants store raphides in mucilaginous environments and also do not taste acrid.
There is some doubt about the role of raphides in defense against herbivory. Insects are only rarely affected by them, and no more than by sand. Ruminants are not harmed by them, because certain bacteria in their rumen help to degrade oxalate.

Plants containing raphides

Common names vary. The following list is incomplete. Raphides are found in many species in the families Araceae and Commelinaceae, but are also found in a few species in a number of other families.
Acanthaceae:

Strobilanthes barbata
Strobilanthes callosa
Strobilanthes ciliata
Strobilanthes heyneana
Strobilanthes integrifolia
Strobilanthes lupulina
Strobilanthes reticulata
Strobilanthes sessilis
Thunbergia erecta
Thunbergia laurifolia

Actinidiaceae:

Actinidia arguta
Actinidia callosa
Actinidia chinensis
Actinidia chrysantha
Actinidia deliiosa
Actinidia eriantha
Actinidia fulvicoma
Actinidia glaucophylla
Actinidia hemsleyana
Actinidia indochinensis
Actinidia lanceolata
Actinidia macrosperma
Actinidia polygama
Actinidia rufa
Clematoclethra scandens
Saurauia pittieri
Saurauia subspinosa

Alstroemeriaceae:

Alstroemeria angustifolia
Alstroemeria aurea
Alstroemeria exserens
Alstroemeria hookeri
Alstroemeria ligtu
Alstroemeria magnifica
Alstroemeria pallida
Alstroemeria pelegrina
Alstroemeria presliana
Alstroemeria pulchra
Alstroemeria versicolor
Bomarea multiflora
Bomarea ovallei

Amaranthaceae:

Beta
Spinacia

Amaryllidaceae:

Crinum amabile
Leucojum aestivum
Pancratium sickenbergeri

Araceae:

Alocasia macrorrhizos
Amorphophallus bulbifer
Amorphophallus blumei
Amorphophallus dunnii
Amorphophallus gallaensis
Amorphophallus konjac
Amorphophallus krausei
Amorphophallus muelleri
Amorphophallus paeoniifolius
Amorphophallus salmoneus
Amorphophallus sutepensis
Amorphophallus taurostigma
Amorphophallus titanum
Amorphophallus variabilis
Anaphyllopsis americana
Anthurium andraeanum
Anthurium acaule
Anthurium crystallinum
Anthurium cubense
Anthurium halmoorei
Anthurium harrisii
Anthurium jenmannii
Anthurium magnificum
Anthurium maricense
Anthurium pallidiflorum
Anthurium pedatoradiatum
Anthurium schlechtendalii
Anthurium signatum
Anthurium spectabile
Anubias heterophylla
Arisaema
Arisarum vulgare
Arum pictum
Caladium
Calla palustris
Cercestis dinklagei
Colocasia affinis
Colocasia gigantea
Colocasia esculenta
Cyrtosperma chamissonis
Cyrtosperma johnstonii
Cyrtosperma merkusii
Dieffenbachia seguine
Epipremnum
Lemna minor
Monstera deliciosa
Philodendron adamantinum
Philodendron bipinnatifidum
Philodendron brasiliense
Philodendron corcovadense
Philodendron dardanianum
Philodendron lundii
Philodendron megalophyllum
Philodendron moonenii
Philodendron oblongum
Philodendron paludicola
Philodendron pedatum
Philodendron petraeum
Philodendron propinquum
Philodendron saxicola
Philodendron speciosum
Philodendron tweedieanum
Philodendron uliginosum
Philodendron undulatum
Philodendron williamsii
Pinellia ternata
Pinellia pedatisecta
Pistia stratiotes
Spathiphyllum
Stenospermation longiopetiolatum
Syngonium schottianum
Typhonium flagelliforme
Xanthosoma sagittifolium
Zantedeschia aethiopia

Araliaceae:

Brassaia

Arecaceae:

Acrocomia aculeata
Acrocomia crispa
Acrocomia emensis
Acrocomia glaucensis
Acrocomia hassleri
Acrocomia intumescens
Acrocomia totai
Allagoptera campestris
Ammandra decasperma
Aphandra natalia
Archontophoenix alexandrae
Areca catechu
Areca macrocalyx
Areca novohibernica
Arenga tremula
Bactris gasipaes
Brassiophoenix dryomphloeides
Butia buenopolensis
Butia campicola
Butia capitata
Butia catarinensis
Butia exospadix
Butia leptospatha
Butia marmorii
Butia microspadix
Butia pubispatha
Calyptrocalyx albertisianus
Calyptrocalyx hollrungii
Calyptrocalyx lepidotus
Calyptrocalyx spicatus
Caryota cumingii
Caryota mitis
Caryota rumphiana
Chamaedorea tepejilote
Cocos nucifera
Copernicia tectorum
Cyrtostachys renda
Drymophloeus pachycladus
Drymophloeus subdistichus
Dypsis decaryi
Dypsis leptocheilos
Dypsis lutescens
Euterpe oleracea
Howea fosteriana
Hydriastele rostrata
Hyophorbe verschaffeltii
Iguanura sp.
Laccospadix australasicus
Linospadix microcaryus
Linospadix monostachyos
Livistonia saribus
Normanbya normanbyi
Oenocarpus bacaba
Oenocarpus bataua
Oncosperma horridum
Oncosperma tigillarium
Pelagodoxa henryana
Pinanga coronata
Pinanga negrosensis
Pinanga malaiana
Pinanga scortechinii
Prestoea acuminata
Ptychosperma caryotoides
Rhapis excelsa
Solfia samoensis

Aspleniaceae:

Asplenium cardiophyllum

Asparagaceae:

Agave debilis
Agave geminiflora
Agave gracillima
Agave guerrerensis
Agave guttata
Agave hauniensis
Agave howardii
Agave longiflora
Agave multicolor
Agave nanchititlensis
Agave neonelsonii
Agave parrasana
Agave platyphylla
Agave polycantha
Agave potosina
Agave pratensis
Agave scabra
Agave singuliflora
Agave stictata
Agave tequilana
Agave victoriae-reginae
Bellevalia paradoxa
Chlorophytum orchidastrum
Cordyline terminalis
Dracaena sanderiana
Hyacinthella nervosa
Hyacinthella hispida
Hyacinthella siirtensis
Hyacinthella venusta
Ornithogalum alpiganum
Ornithogalum caudatum
Polygonatum odoratum
Polygonatum prattii
Triteleia crocea
Triteleia dudleyi
Triteleia hendersonii
Triteleia hyacintha
Triteleia ixioides
Triteleia laxa
Yucca treculiana

Asphodelaceae:

Aloe harlana
Aloe nobilis
Asphodelus aestivus
Gasteria obliqua
Haworthia cymbiformis

Asteraceae:

Aster warmingii
Baccharis glaziovii
Silybum marianum
Solidago chilensis

Balsaminaceae:

Impatiens arguta
Impatiens balfourii
Impatiens niamniamensis
Impatiens parviflora
Impatiens thomsonii

Boraginaceae:

Onosma mollis
Onosma sorgeri

Bromeliaceae:

very widespread

Cactaceae:

widespread

Colchicaceae:

Colchicum kurdi

Commelinaceae:

Cochliostema odoratissimum
Commelina coelestis
Commelina tuberosa
Dichorisandra thyrsiflora
Tinantia anomala
Tinantia pringlei
Tradescantia ohiensis
Tradescantia pallida
Tradescantia spathacea
Triceratella

Cucurbitaceae:

Momordica dioica
Momordica cochinchinensis

Dioscoreaceae:

Dioscorea bulbifera
Dioscorea esculenta
Dioscorea hispida
Dioscorea japonica
Dioscorea polystachya

Ebenaceae:

Diospyros montana

Euphorbiaceae:

Ricinus communis

Fabaceae:

Acacia robeorum
Medicago truncatula
Pachyrhizus tuberosus

Haemodoraceae:

Anigozanthos flavidus
Conostylis candicans

Heliconiaceae:

Heliconia indica
Heliconia longiflora

Hydrangeaceae:

Hydrangea macrophylla

Lamiaceae:

Satureja khuzestanica

Liliaceae:

Drimia maritima

Lowiaceae:

Orchidantha maxillarioides

Marcgraviaceae:

very widespread

Musaceae:

Musa acuminata
Musa ingens
Musa maclayi

Nyctaginaceae:

Bougainvillea glabra
Bougainvillea spectabilis

Oleaceae:

very widespread

Onagraceae:

Epilobium tibetanum
Ludwigia peploides

Orchidaceae:

very widespread

Pandanaceae:

Pandanus dubius
Pandanus leram
Pandanus tectorius

Passifloraceae:

Passiflora cincinnata

Philydraceae:

Heimholtzia glaberrima

Piperaceae:

Manekia incurva
Manekia sydowii
Peperomia fagerlindii
Peperomia incana
Peperomia longespicata
Peperomia macrostachya
Peperomia serpens
Peperomia vinasiana
Piper augustum
Piper betle
Piper caldense
Piper cavendishioides
Piper chrysostachyum
Piper cinereum
Piper confusum
Piper dotanum
Piper eriopodon
Piper filistium
Piper glabrescens
Piper hispidum
Piper lapathifolium
Piper leucophyllum
Piper mikanianum
Piper munchanum
Piper corrugatum
Piper peltatum
Piper pseudovariabile
Piper sancti-felicis
Piper umbellatum
Piper urdanetanum
Piper oxystachyum
Zippelia begoniifolia

Poaceae:

Cenchrus echinatus

Polygonaceae:

Rheum rhabarbarum

Pontederiaceae:

Monochoria africana
Pontederia cordata
Pontederia crassipes

Roridulaceae:

Roridula gorgonias

Rosaceae:

Rubus alutaceus

Rubiaceae:

very widespread

Tetrameristaceae:

Pelliciera rhizophorae
Pentamerista neotropica
Tetramerista glabra

Typhaceae:

Typha angustifolia
Typha latifolia

Vitaceae:

Cissus alata
Cissus discolor
Cissus quadrangularis
Cissus repanda
Cissus rotundifolia
Parthenocissus quinquefolia
Parthenocissus tricuspidata
Parthenocissus vitacea
Rhoicissus tomentosa
Tetrastigma voinierianum
Vitis berlandieri
Vitis rupestris
''Vitis vinifera''